I recently purchased a 1988 Starcraft that has been used on only freshwater. I plan on using on Saltwater. I am restoring the boat and have removed the console, the floor, and he old old old flotation foam (which released a lot of water while being removed). With all of the water trapped under the foam I am hesitant going back with foam because I do not want any saltwater getting trapped between the foam and the aluminum. I plan on coating the inside of the hull with Gluvit and having the exterior bottom done professionally. What type of flotation material should I go back with if my goal is not to allow any water to become trapped under the foam?

The solution is to keep (fresh or salt) water from getting the foam wet. If it is not possible to keep the bulge dry, remove the foam. That is the only long term solution. At the end of the season wash the bulge out using fresh water, and salt-away to remove salt build up, and dry it out before putting it away.

KenO, a couple of considerations regarding foam in your hull.
#1 if the hull is longer than 20'LOA even 20'-1"; then foam is not required for level flotation, as near as I understand the CFR?
#2 (if you need the foam for your own reasons or circumstances?) you mentioned coating your hull inside (?)- if that work is done by etching with acid, dilution rinsing and then chromate conversion while wet- followed by primer and finally some form of topcoat ?? THEN the water held in close contact to the coating by foam material won't be reactive with the metal. The only way the bilge water would be reactive is if the metal is bare or the paint coating is compromised, or you introduced large or small ph solutions into the bilge.
#3 if you're adding foam, then styrene types are much less reactive by themselves- they're more inert. So you can buy the blue or pink 'bury-able' foam sheeting and cut pieces to fit and stack them under a ply deck- with minimum chemical reaction.

However, even a styrene plastic can form a thin film of water to metal if they're in close contact for palm or larger size area-& that can lead to a corrosion cell. Running 'ribs' of silicone glue on the foam panels - letting it dry before installing would put a small set of 'ribs' on the foam resulting very small areas of contact- so the points of contact are not wide flat sheet areas. That would allow the bilge water to move and therefore stay aerated enough to remain neutral ph. For example; Chaps suggested ping pong balls- they'd all have very small points of contact so there would be small chance of forming acid cells due the geometry of the two shapes' intersection- not much water volume in the area of contact.

I'm not sure how well any foam stands up to fuel in the bilge? So I'm not remarking about that aspect of material selection.

Not sure I'm any help in speculating about flotation foam? IMO, these considerations are valid when planning to install foam in your boat's bilge.